Knowledge of ion species and their concentrations in plasmas is necessary to the conduct of research and to improve manufacturing processes which utilize plasmas. The mass spectrometers currently used for this purpose are complex, expensive, and usually too large to properly characterize a plasma. These spectrometers can be used only for edge studies, in which measurements are made at the edges of a plasma rather than at locations where articles are placed for processing. Edge spectroscopy and theoretical modeling provide useful qualitative information on plasma composition, but normally do not provide quantitative information. The present invention provides an inexpensive and less complex mass spectrometer which is sufficiently small that it is capable of use in characterizing the entire volume of a plasma without perturbing it, that is, without significantly changing the characteristics of a plasma by use of the spectrometer. It is capable of use for performing in-situ mass spectroscopy of plasmas. Quadrupole mass spectrometers are widely used for characterization of plasmas at their edges. The cost of a quadrupole mass spectrometer and necessary accessories suitable for such use is in the range of $30,000 to $100,000.00. An instrument of the present invention can be fabricated and the required power supply and signal processing apparatus purchased for less than $1,000.00 (the cost will increase with increased sophistication of the electronics).
A plasma is comprised of atoms and molecules in a gaseous state having no electrical charge, ions formed from the gas by providing energy to the gas, and electrons. The un-ionized atoms and molecules are termed neutrals. The ions are normally positive, but attachment of electrons to neutrals can take place so that negative ions are present in the plasma. A plasma may be produced in a chamber maintained at low pressure (normally, at pressures in the vacuum range) by introducing a gas into the chamber and providing energy to the gas by such means as an arc discharge or radio-frequency induction fields.
Plasma processing is currently used primarily in manufacture of microelectronics components and is expected to be used more extensively in the future. Wet chemical processes are used to etch (remove material from) surfaces of wafers which are an intermediate product in the manufacture of integrated circuit devices, or chips. Waste products from such wet chemical processing are hazardous and expensive to reclaim. Plasma processing may be substituted for wet chemical processing. An oxygen plasma may be used for removal of photoresist mask material from microelectronics components after the process requiring masking is accomplished. Cleaning of surfaces may be done by plasma processing. For example, cutting fluids, oils, and greases may be removed from machined parts by means of an oxygen plasma, thus avoiding use of solvents. When a part is exposed to the plasma, oxygen ions strike the surface of the part, breaking chemical bonds between atoms of the hydrocarbons and creating reactive sites. Also, the plasma provides energy to the near-surface region which assists desorption of contaminants and enhances the chemical reactions. The products of this cleaning process are carbon dioxide and water vapor. Plasma processing is used to deposit material on a surface. For example, a mirror may be coated with a reflecting layer of gold or aluminum. Glass may be coated with a material which prevents ultraviolet light from passing through the glass. An oxygen plasma may be used to produce a thin layer of silicon oxide on a silicon surface. Diamond-coated objects may be produced by subjecting the object to carbon ions produced in a plasma. implantation of ions in a surface to modify its properties may be accomplished by plasma processing. For example, surfaces may be hardened by implanting nitrogen ions (nitriding) or boron atoms (boriding). The mass spectrometer of this invention provides means to monitor such processes in a production facility and to study such processes.